JDO : Fetch Groups

When an object is retrieved from the datastore by JDO typically not all fields are retrieved immediately.
This is because for efficiency purposes only particular field types are retrieved in the initial access
of the object, and then any other objects are retrieved when accessed (lazy loading).
The group of fields that are loaded is called a fetch group.
There are 3 types of "fetch groups" to consider

Default Fetch Group : defined in all JDO specs, containing the fields
of a class that will be retrieved by default (with no user specification).

Named Fetch Groups : defined by the JDO2 specification, and defined
in MetaData (XML/annotations) with the fields of a class that are part of that fetch group.
The definition here is static

The fetch group in use for a class is controled via the FetchPlan
interface. To get a handle on the current FetchPlan we do

FetchPlan fp = pm.getFetchPlan();

Default Fetch Group

JDO provides an initial fetch group, comprising the fields that will be retrieved when an object
is retrieved if the user does nothing to define the required behaviour. By default the default
fetch group comprises all fields of the following types :-

By default the FetchPlan will include the default fetch group. We have changed this above by
adding the fetch group "otherfield", so when we retrieve an object using this
PersistenceManager we will be retrieving the fields name AND other since they
are both in the current FetchPlan. We can take the above much further than what is shown by
defining nested fetch groups in the MetaData. In addition we can change the FetchPlan just
before any PersistenceManager operation to control what is fetched during that operation.
The user has full flexibility to add many groups to the current Fetch Plan.
This gives much power and control over what will be loaded and when. A big improvement over JDO 1.0

The FetchPlan applies not just to calls to PersistenceManager.getObjectById(), but also
to PersistenceManager.newQuery(), PersistenceManager.getExtent(),
PersistenceManager.detachCopy and much more besides.

Dynamic Fetch Groups

The mechanism above provides static fetch groups defined in XML or annotations.
That is great when you know in advance what fields you want to fetch. In some situations
you may want to define your fields to fetch at run time. This became standard in JDO2.2
(was previously a DataNucleus extension). It operates as follows

import org.datanucleus.FetchGroup;
// Create a FetchGroup on the PMF called "TestGroup" for MyClass
FetchGroup grp = myPMF.getFetchGroup(MyClass.class, "TestGroup");
grp.addMember("field1").addMember("field2");
// Add this group to the fetch plan (using its name)
fp.addGroup("TestGroup");

So we use the DataNucleus PMF as a way of creating a FetchGroup, and then register that
FetchGroup with the PMF for use by all PMs. We then enable our FetchGroup for use in the
FetchPlan by using its group name (as we do for a static group). The FetchGroup allows you
to add/remove the fields necessary so you have full API control over the fields to be
fetched.

Fetch Depth

The basic fetch group defines which fields are to be fetched. It doesn't explicitly define how far
down an object graph is to be fetched. JDO2 provides two ways of controlling this.

The first is to set the maxFetchDepth for the FetchPlan. This value specifies how
far out from the root object the related objects will be fetched. A positive value means that
this number of relationships will be traversed from the root object. A value of -1 means that
no limit will be placed on the fetching traversal. The default is 1. Let's take an example

A further refinement to this global fetch depth setting is to control the fetching of recursive
fields. This is performed via a MetaData setting "recursion-depth". A value of 1 means that only
1 level of objects will be fetched. A value of -1 means there is no limit on the amount of recursion.
The default is 1. Let's take an example

So when we fetch a Directory, it will fetch 2 levels of the children field, hence fetching
the children and the grandchildren.

Fetch Size

A FetchPlan can also be used for defining the fetching policy when using queries. This can be
set using

pm.getFetchPlan().setFetchSize(value);

The default is FetchPlan.FETCH_SIZE_OPTIMAL which leaves it to DataNucleus to optimise the fetching
of instances. A positive value defines the number of instances to be fetched. Using
FetchPlan.FETCH_SIZE_GREEDY means that all instances will be fetched immediately.